The invention relates to packet based communications and to a format of transmission frames used in such communications.
Packet based communications can be used, e.g., in radio-based cellular systems, where mobile users are connected via wireless links to fixed base stations. In packet based communications, digital information packets are transmitted without rigid allocation of "time slots" to users.
Various protocols and formats have been proposed for transmission frames, some of which are reviewed as follows:
U.S. Pat. No. 4,649,543, issued Mar. 10, 1987 to Levine, discloses a radiotelephone system in which data frames include data portions which are interleaved with so-called SAT/SYNC words. Each such word is a 21-bit correlator word which uniquely provides combined radio frequency (RF) frame synchronization, digital SAT (supervisory tone) information, and system state information. After a subscriber unit has requested a digital channel for making a call through a fixed site, this site transmits data frames to the subscriber unit with "dotting" information (001100110011 . . . ) for bit synchronization. If a digital channel to be assigned to the subscriber unit is not the same as the channel currently used by the subscriber unit, a handoff command is transmitted from the fixed site to the subscriber unit to switch to a channel capable of the digital service. After the call is set up, the data portion is used to transmit digitally encoded speech at an effective rate of 9.6 Kb/s.
U.S. Pat. No. 5,237,570, issued Aug. 17, 1993 to Smolinske et al., discloses a radiotelephone data transfer system in which a down-link message to a mobile consists of six fields termed synchronization, acknowledgement, time slot number, register select, destination address and data fields. An up-link message by a mobile consists of four fields, namely priority, register select, source address and data fields.
U.S. Pat. No. 4,807,222, issued Feb. 21, 1989 to Amitay, discloses scanning of a portion of a transmission frame to determine whether the frame is occupied by a packet.
U.S. Pat. No. 4,789,983, issued Dec. 6, 1988 to Acampora et al. and the paper by Z. Zhang et al., "Performance of a Modified Polling Strategy for Broadband Wireless LANS in a Harsh Fading Environment", Telecommunications Systems, Vol. 1 (1993), pp. 279-294 disclose packet based communications with user devices being polled for information regarding communications needs.
The following are concerned with mobile radio systems using time-multiplexed transmission, and are included here as being of general interest. Particular time frame formats are disclosed.
K. Zhang et al., "An Integrated Voice/Data System for Mobile Indoor Radio Networks Using Multiple Transmission Rate," IEEE Global Telecommunications Conference and Exhibition, Globecom '89, vol. 3, pp. 1366-1370 discloses a time multiplexing technique in which each time frame is divided into time slots for communicating voice and data. The time slots have different durations, corresponding to two different transmission rates.
U.S. Pat. No. 5,199,031, issued Mar. 30, 1993 to Dahlin, discloses a technique for time division of radio traffic and control channels for communications between a mobile station and an associated base station.
European Patent Document No. 0,286,614 discloses a technique for dividing a radio traffic channel into time slots, and a radio control channel into superframes, each comprising multiple frames of time slots.
U.S. Pat. No. 4,972,506, issued Nov. 20, 1990 to Uddenfeldt, discloses a TDMA technique in which allocated but unoccupied time slots are used to reduce the bit rate, for increased transmission range between mobile and base stations.
The paper entitled "Funkfernsprechen Bitwise" Funkschau 24/1986, pp. 50-52, discloses a cellular system for digital wireless communications. FIG. 2 of the paper shows a time frame and communications channel scheme for the system.
The invention described below provides for a particularly convenient transmission frame format for packet based communications.